In the field of industrial radiography, especially for non-destructive testing applications, film materials showing excellent image quality after processing in hardener containing developer and/or fixer solutions are well-known. The said materials are characterised by the presence of high coated amounts of silver bromoiodide emulsion crystals in the range of 6 to 20 grams, expressed as the equivalent amount of silver nitrate, per square meter and per side. Long processing times, varying between 5 and 15 minutes, are time consuming and shorter processing times are not available without drying problems (e.g. sticking) and degeneration of image quality (e.g. deposit on the film due to "pi-line" defects) and sensitometry (e.g. too low a contrast and/or a speed).
In the field of industrial radiography, and especially for non-destructive testing applications, any time saving measure is welcome: the tendency is to reduce the processing time to a maximum of 5 minutes and more preferably to about 2.5 minutes. In order to achieve high film speed, which is an indispensible asset especially for direct-X-ray applications, efficient absorption of the exposure radiation is a prime condition. It has been shown empirically that for X-rays the mass absorption coefficient is proportional to a power of the atomic number Z as has been described in the "Encyclopaedic Dictionary of Physics" vol. 7, p. 787, eq. 10, Ed. J. Thewlis, Pergamom Press, Oxford 1957. This strongly disfavours e.g. the use of chloride (Z=17) compared to bromide (Z=35) or iodide (Z=54). Otherwise a cubic habit is favoured because of the volume effect of the silver halide crystal to get good absorption properties when exposing said crystals.
Nevertheless it has been shown in EP-A No. 91202761, filed Oct. 24, 1991 that the object to get rapid processable industrial radiographic films can be accomplished by a photographic silver halide material comprising a film support and on one or both sides thereof at least one silver halide emulsion layer which is characterised in that each silver halide emulsion layer comprises as silver halide silver chloride or silver chlorobromide the amounts of bromide being at most 25 mole %, in that it has a gelatin to silver halide (expressed as silver nitrate) ratio by weight from 3:10 to 6:10 and an amount of silver halide corresponding to from 5 g to 15 g of silver per sq. m. and in that the photographic material has been fore-hardened to an extent such that when it is immersed in demineralised water of 25.degree. C. for 3 minutes there is absorbed less than 2.5 g of water per gram of gelatin.
However as it is nowadays required for the said industrial radiographic films to be processable in different processing cycles within a total processing time in the range between 2.5 and 8 minutes, the problem arises that the silver chloride and/or silver chlorobromide emulsions used in the said EP-Application may show too high, irreproducible fog levels so that the compatibility in different processing cycles is not guaranteed.
Due to the solubility of crystals rich in silver chloride and to the large amount of said crystals in the coated layers of industrial radiographic material silver sludge appears. Sulphite rich developers further reinforce the amount of silver sludge. As a result, so-called "pi-line" defects may occur, as a result of unwanted deposit of silver sludge on the rollers in the developer racks of automatic processors that is carried over to the film at recurrent distances. Consequently an enhanced maintenance frequency is necessary for the processor resulting in an enhanced cost and loss of time.